The present invention relates generally to mechanical displacement or vibration compensation systems, and, more particularly relates to such systems employing feedback control.
In any mechanical system involving motion there is a potential for excessive displacement or vibration relative to limits designed into such system. For example, as any human traveler has observed while on movable systems such as spacecraft, aircraft, seacraft, motor vehicles, elevators etc., excessive displacement or vibration not only can contribute to one""s fatigue and discomfort, but, if such limits are exceeded where impact occurs, can also pose serious safety risks derived from potential damage to the system""s structure. These transportation systems are not the only class of mechanical systems where excessive vibration can be problematic. There are other classes of mechanical systems having potential for excessive displacement or vibration such as, for example, machinery support systems used in non-transportation applications such as factory or manufacturing environments, although certain of such machinery support systems can also be employed in the aforementioned transportation applications.
Machinery support systems are typically designed to provide vibration isolation between the machinery and its foundation so as to prevent transmission of machinery forces into the foundation where they can generate vibration and sound. This design requirement conflicts with the additional need to maintain a specified clearance within the mount. Maintaining clearances within the mount is necessary so as to minimize likelihood of the machine coming into hard contact with its foundation. The specified clearance is likely to be exceeded when the mount foundation is subjected to low frequency high amplitude displacements, such as might occur in a ship in heavy seas, a submarine during maneuvers, a land vehicle on a rough road, an aircraft during maneuvering or an aircraft flying in turbulence. Hard contact will, in most if not all circumstances, adversely affect vibration isolation performance of the mount. In addition, impact between mount and foundation could result in damage to the machine or its foundation, posing potentially serious safety risks to passengers and others aboard transportation systems utilizing any such mount/foundation vibration isolation system, as noted above.
To maintain clearance, a compensation system is often employed in conjunction with or as part of the mounting system. In essence, prior art compensation systems seek to stiffen the mount, because a stiff mount (in contrast with a compliant mount) will allow for little relative motion between machine and foundation which reduces likelihood of impact therebetween. However, the stiffer the mount is made, the worse the isolation becomes. An effective vibration isolation system performs best if the mount is very compliant which allows for significant motion between the machine and its foundation. In a particular circumstance, if foundation motions are relatively severe and clearance requirements are relatively stringent so that only relatively small displacements between machine and foundation are allowed, then a fixed prior-art compensation system could impair vibration isolation performance of the mounting system, because the compensation system will, effectively, have to make the mount much stiffer than it would otherwise have made it.
Compensation systems in the prior art are typically linear feedback control systems which feedback deflection (displacement), velocity or acceleration across the mount and generate forces across the mount to maintain a minimum clearance. Since these systems cannot adjust themselves to their environment (foundation motion) they must be designed for worst case environment which can lead to a larger than necessary adverse impact on vibration isolation performance when the environment is not worst case.
What is needed, and what the prior art does not offer, is a dynamic compensation system which can control the degree of vibration isolation as a function of changing environment to ensure that impact does not occur, while, at the same time, maximizing vibration isolation. The present invention offers such a welcome solution to the problems and shortcomings of the prior art.
Embodiments of the present invention relate to systems for controlling motion of a first object movably mounted within a second object. In such systems, stiffness of the movably mounted structure supporting the first object is established. An allowable maximum range of motion of the first object relative to the second object is established. Displacement or deflection of the first object relative to the second object is continuously estimated. And, the stiffness is adjusted in a manner to prevent the maximum displacement from exceeding the allowable maximum range of motion, in response to the maximum displacement being estimated to be beyond said allowable maximum range.
Further features of these embodiments of the present invention include the first object as a ship""s deck and the second object as a ship""s hull, and where the stiffness is established through a pneumatic system. In these further features, acceleration sensors measure hull acceleration, relative displacement sensors measure deck to hull relative displacement, and relative velocity sensors measure deck to hull relative velocity. These measurements are used in a feedback loop control system to continuously calculate estimated values of maximum displacement and to vary pressure of the pneumatic system to compensate accordingly.
In another aspect, the present invention is a new approach for actively maintaining clearances in a vibration isolation mount with minimum impact on vibration isolation performance when the mount foundation is subjected to large amplitude low frequency vibration. The approach uses measurements of acceleration of mount foundation, displacement across the mount and velocity across the mount to predict time history of relative displacement between mount and foundation. The prediction is continuously updated to account for any changes that occur in foundation acceleration. If predicted relative displacement exceeds the maximum allowable value, the control system increases mount stiffness until the prediction indicates that relative displacement will be less than the maximum allowable. In the case of an air mount, the pumping of air directly into the mount employing an air cylinder or hydraulic cylinder or other vibration absorbing mechanism can increase stiffness. For other types of mounting systems, e.g., a metal spring or an elastomeric mount, a separate actuator is provided such as an electromagnetic actuator or an electrodynamic actuator, to be used with either such spring or such mount alone, or possibly together in combination with the aforementioned air cylinder, hydraulic cylinder or other vibration-absorbing mechanism or entity.
It is thus a general object of the present invention to provide an improved mechanical displacement or vibration compensation system employing feedback control.
It is a further object of the present invention to provide such compensation system having such control which continuously estimates maximum displacement and adjusts stiffness of the system to maintain maximum displacement within an allowable maximum range.
It is thus advantageous to employ embodiments of the present invention in mechanical systems involving motion. This Predictive Active Compensation Systems (PACS) approach minimizes the stiffening effect by stiffening the mount only as much as is needed to ensure that maximum allowable relative displacement between machine and foundation will not be exceeded. Should foundation acceleration decrease, stiffness will be reduced accordingly. This approach offers the possibility of using much more compliant mounting systems than could heretofore be imagined because of the possibility of their bottoming out when operating in any real environment. PACS, when operating with the environment in a quiescent state, offer substantial improvement in vibration isolation performance. These systems thus offer many advantages as they are employable in a wide variety of applications where vastly improved noise and vibration reductions are needed.
In other words, an advantage of this approach is that the mount is made only as stiff as necessary to ensure that the machine to foundation relative displacements do not exceed a maximum allowable value. Since the mount stiffness is kept as small as possible, the mounting system will provide the best possible vibration isolation performance consistent with maintaining the desired mount clearances in whatever environment it encounters. This is especially important in quiescent environments (i.e. conditions under which little or no change in mount clearance occurs) where mount stiffness can therefore be kept very low (low stiffness is equivalent to highly compliant, and high stiffness is equivalent to almost non-compliant). The concept offers the possibility of using mounting system with much lower stiffness and much better vibration isolation performance than have heretofore been contemplated because of the tendency of these very compliant mounting systems to bottom out.
Further objects and advantages of the present invention will be appreciated after reviewing the detailed description of the preferred embodiments hereinbelow in combination with the appended drawings wherein: